1. Field of the Invention
The present invention relates to a modulation apparatus for performing phase modulation using a PLL (Phase-Locked-Loop) circuit. More particularly, the present invention relates to a modulation apparatus capable of correcting the non-linearity of a voltage controlled oscillator used in a PLL circuit.
2. Description of the Background Art
A polar modulation system is one of wireless communications systems which perform phase modulation using a PLL circuit. FIG. 20 is a block diagram showing an exemplary functional configuration of a polar modulation apparatus 900 used in a transmission apparatus of a conventional polar modulation system. See FIG. 1 of “SIMPLE POLAR-LOOP TRANSMITTER FOR DUAL-MODE BLUETOOTH”, Takashi Oshima and Masaru Kokubo, Hitachi Ltd., Central Research Laboratory, Circuits and Systems, 2005. ISCAS 2005. IEEE International Symposium on 23-26 May 2005 Page(s):3966-3969 Vol. 4 (Non-Patent Document 1).
In FIG. 20, the polar modulation apparatus 900 comprises an I/Q conversion section 901, a polar conversion section 902, an AM modulation section 903, a PM modulation section 904, and a power amplifier (PA) 905. Transmission data containing information to be transmitted is input to the I/Q conversion section 901. The I/Q conversion section 901 converts the transmission data into an in-phase component and a quadrature component and outputs these components as a digital I signal and Q signal. The polar conversion section 902 converts the I and Q signals output by the I/Q conversion section 901 into an amplitude component and a phase component in a polar coordinate system and outputs these components as a digital amplitude signal R and a phase signal θ. The AM modulation section 903 converts the amplitude signal R into an analog value and inputs the analog value to the PA 905. The PM modulation section 904 comprises a PLL circuit including a voltage controlled oscillator, and changes a phase of an output signal of the voltage controlled oscillator based on the phase signal θ and inputs the phase-modulated signal to the PA 905. The PA 905 modulates an amplitude of the signal output from the PM modulation section 904 based on the signal output from the AM modulation section 903 and outputs the resultant transmission signal to an antenna. Thereby, the transmission data is polar-modulated and is then transmitted from the antenna.
To achieve a broad band in the PM modulation section 904, a system called a two-point modulation method has been proposed. FIG. 21 is a diagram showing a detailed exemplary configuration of the conventional PM modulation section 904 employing the two-point modulation method. See FIG. 2 of Non-Patent Document 1. In FIG. 21, the PM modulation section 904 comprises a phase comparator (PFD) 906, a charge pump (CP) 907, a low-pass filter (LPF) 908, a voltage controlled oscillator (VCO) 909, a frequency divider (1/N) 910, a ΣΔ converter 911, a digital analog converter (DAC) 912, a differentiator (d/dt) 913, and an adder 914.
The phase comparator 906, the charge pump 907, the low-pass filter 908, the voltage controlled oscillator 909, and the frequency divider 910 constitute a PLL circuit. In the two-point modulation method, a frequency division ratio of the frequency divider 910 is changed based on the phase signal θ, and a control voltage applied to the voltage controlled oscillator 909 is changed based on the phase signal θ. Thus, in the two-point modulation method, a signal propagating through the PLL circuit is changed between before and after the low-pass filter 908.
The phase signal θ is differentiated by the differentiator 913 and is converted into a ΣΔ-converted analog signal by the ΣΔ converter 911 and the ΣΔ-converted analog signal is then input as a signal IN1(t) to the frequency divider 910. Phase modulation is achieved by modulating the frequency division ratio using the signal IN1(t). It is known that a frequency response from an input of the frequency divider 910 to an output of the voltage controlled oscillator 909 is a low-pass response (see expression (1) in Non-Patent Document 1).
On the other hand, the phase signal θ which has been converted into an analog signal by the digital-analog converter 912 is input as a signal IN2(t) to the adder 914. Phase modulation is also achieved by modulating the control voltage using the signal IN2(t). It is known that a frequency response from a control terminal to an output of the voltage controlled oscillator 909 is a high-pass response (see expression (3) in Non-Patent Document 1).
Thus, when the two-point modulation method is used, a combination of phase modulations having a low-pass response and a high-pass response can be provided before and after the low-pass filter 908, thereby achieving a broad-band phase modulation in the PM modulation section 904.
As described above, it is known that, when the two-point modulation method is used, broad-band phase modulation is achieved. However, the voltage controlled oscillator in the PLL circuit often has non-linearity. Therefore, when a non-linear voltage controlled oscillator is used, a distortion is likely to occur in an output, so that desired phase modulation may not be achieved. Therefore, even when the two-point modulation method is used, the problem with the non-linearity of the voltage controlled oscillator needs to be solved.
The non-linearity of the voltage controlled oscillator sometimes causes a problem. For example, Japanese Patent Laid-Open Publication No. 7-55924 (Patent Document 1) and Japanese Patent Laid-Open Publication No. 10-115677 (Patent Document 2) disclose inventions which solve the non-linearity of the voltage controlled oscillator. Specifically, Patent Documents 1 and 2 propose a technique of calculating characteristics inverse to the non-linear characteristics of the voltage controlled oscillator, and based on the obtained inverse characteristics, correcting the control voltage.
FIGS. 22A to 22C are diagrams for describing the conventional technique of calculating the inverse characteristics of the voltage controlled oscillator and correcting a control voltage. As shown in FIG. 22A, the voltage controlled oscillator is assumed to have characteristics A1 as characteristics of an output frequency with respect to a control voltage. The characteristics A1 are non-linear. In this case, if the control voltage is changed in accordance with a sine curve, the output frequency is distorted. Therefore, in the conventional correction technique, as shown in FIG. 22B, an inverse function of the characteristics A1 is calculated as inverse characteristics IA1. In this case, a voltage controlled oscillator having the inverse characteristics IA1 is assumed, and characteristics of a distorted output frequency obtained when a sine curve is input are calculated as characteristics B1 of a control voltage after correction. A control voltage before correction is associated with a control voltage after correction as shown in FIG. 22B, thereby correcting a control voltage. For example, as shown in FIG. 22C, when a sine-curve variation is provided as a control voltage before correction, then if the characteristics B1 are used as variations in a control voltage after correction, a sine curve having no distortion is output from the voltage controlled oscillator.
However, when the technique of calculating the inverse characteristics of the voltage controlled oscillator to correct a control voltage is employed, an advanced digital signal processing technique capable of achieving a Fourier transform is required to calculate the inverse characteristics of the voltage controlled oscillator, for example.
Also, the correction of the non-linearity of the voltage controlled oscillator largely relates to modulation of a transmission signal, and it is necessary to remove a distortion of the voltage controlled oscillator more quickly. However, when the advanced digital signal processing technique is used to correct the non-linearity of the voltage controlled oscillator, it is difficult to achieve the correction quickly. When the digital signal processing technique is used, a problem inevitably arises with cost.
Even when the inverse characteristics of the voltage controlled oscillator are previously obtained, the non-linearity can be corrected. However, the characteristics of the voltage controlled oscillator vary over time. If the inverse characteristics are fixed, a deterioration disadvantageously occurs in communications quality.